Editing Bee (section)

===Haplodiploid breeding system===
{{further|Haplodiploidy}}
[[File:Wasp attack.jpg|thumb|left|upright=1.3<!--size for low image-->|Willing to die for their sisters: worker honey bees killed defending their hive against [[yellowjacket]]s, along with a dead yellowjacket. Such [[altruism|altruistic]] behaviour may be favoured by the [[haplodiploid]] [[Sex-determination system|sex determination]] system of bees.]]

According to [[inclusive fitness]] theory, organisms can gain fitness not just through increasing their own reproductive output, but also that of close relatives. In evolutionary terms, individuals should help relatives when ''Cost < Relatedness * Benefit''. The requirements for eusociality are more easily fulfilled by [[haplodiploid]] species such as bees because of their unusual relatedness structure.<ref name=Hughes2008>{{Cite journal | last1=Hughes | first1=W. O. H. | last2=Oldroyd | first2=B. P. | last3=Beekman | first3=M. | last4=Ratnieks | first4=F. L. W. | title=Ancestral Monogamy Shows Kin Selection is Key to the Evolution of Eusociality | doi=10.1126/science.1156108 | journal=Science | volume=320 | issue=5880 | pages=1213–1216 | year=2008 | pmid= 18511689| bibcode=2008Sci...320.1213H | s2cid=20388889 }}</ref>

In haplodiploid species, females develop from fertilized eggs and males from unfertilized eggs. Because a male is [[haploid]] (has only one copy of each gene), his daughters (which are [[diploid]], with two copies of each gene) share 100% of his genes and 50% of their mother's. Therefore, they share 75% of their genes with each other. This mechanism of sex determination gives rise to what [[W. D. Hamilton]] termed "supersisters", more closely related to their sisters than they would be to their own offspring.<ref name="Hamilton1964II">{{cite journal | last=Hamilton | first=W. D. | title=The Genetical Evolution of Social Behaviour II | journal=Journal of Theoretical Biology | date=20 March 1964 | volume=7 | issue=1 | pages=17–52 | doi=10.1016/0022-5193(64)90039-6 | pmid=5875340| bibcode=1964JThBi...7...17H }}</ref> Workers often do not reproduce, but they can pass on more of their genes by helping to raise their sisters (as queens) than they would by having their own offspring (each of which would only have 50% of their genes), assuming they would produce similar numbers. This unusual situation has been proposed as an explanation of the multiple (at least nine) evolutions of eusociality within [[Hymenoptera]].<ref name="Hughes" /><ref>{{cite book |last1=Gullan |first1=P. J. |last2=Cranston |first2=P. S. |title=The Insects: An Outline of Entomology |date=2014 |publisher=Wiley Blackwell |isbn=978-1-118-84615-5 |edition=5th |pages=328, 348–350}}</ref>

Haplodiploidy is neither necessary nor sufficient for eusociality. Some eusocial species such as [[termites]] are not haplodiploid. Conversely, all bees are haplodiploid but not all are eusocial, and among eusocial species many queens mate with multiple males, creating half-sisters that share only 25% of each other's genes.<ref name="NTW">{{cite journal | last=Nowak | first=Martin | author2=Tarnita, Corina | author3=Wilson, E.O. | title=The evolution of eusociality | journal=Nature | year=2010 | volume=466 | pmid=20740005 | issue=7310 | pages=1057–1062 | doi=10.1038/nature09205 | pmc=3279739| bibcode=2010Natur.466.1057N }}</ref> But, monogamy (queens mating singly) is the ancestral state for all eusocial species so far investigated, so it is likely that haplodiploidy contributed to the evolution of eusociality in bees.<ref name="Hughes">{{cite journal | author1=Hughes, William O. H. | author2=Oldroyd, Benjamin P. | author3=Beekman, Madeleine | author4=Ratnieks, Francis L. W. | title=Ancestral Monogamy Shows Kin Selection Is Key to the Evolution of Eusociality | journal=[[Science (journal)|Science]] | volume=320 | issue=5880 | pages=1213–1216 | publisher=American Association for the Advancement of Science | date=May 2008 | doi=10.1126/science.1156108 | pmid=18511689| bibcode=2008Sci...320.1213H | s2cid=20388889 }}</ref>